Multivoice signal switching circuit

ABSTRACT

A multivoice signal switching circuit is incorporated in a circuit for demodulating a multivoice signal containing a main channel signal, a first subchannel signal, and a second subchannel signal, the latter two signals being treated for noise reduction. The demodulating circuit comprises a first circuit for extracting a main channel signal from the multivoice signal, a second circuit for extracting a first subchannel signal from the multivoice signal, and a third circuit for extracting a second subchannel signal. The above switching circuit receives the first and second subchannel signals and selectively outputs one of the two subchannel signals, to a decoder where the original signal is restored from the noise reduction treated signal. This signal is fed to an output circuit means together with the outputs of the first circuit, so that an audio signal is reproduced. The switch circuit has a manual switch for selecting either of the first and second subchannel signals, and is adapted to invalidate selection of the second subchannel signal by the manual switch when the second subchannel signal cannot be sensed in the multivoice signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching circuit for use inmultivoice signal demodulating circuit, and more specifically to amultivoice signal demodulating circuit for selectively outputting anaudio signal from a multivoice signal including at least a main channelsignal, a first subchannel signal and a second subchannel signal.

2. Description of Related Art

At present, in television broadcastings, multivoice signals aretransmitted which include a main channel signal, a first subchannelsignal and a second subchannel signal called "Second Audio Program"(SAP) signal. In this case, the sum of the left- and right-hand audiosignals (L+R) and their difference (L-R) are ordinarily placed on themain channel signal and the subchannel signal, respectively, for astereophonic sound tranmission and reproduction. In addition, the SAPsignal is used to transmits a second language signal in a bilingualbroadcasting.

In the above triple-audio broadcasting, the (L-R) signal and the SAPsignal are respectively suppressed for noise reduction, for example, bya DBX noise reduction system which compresses the dynamic range at alllevels and frequencies. Thereafter, subcarriers for the first and secondsubchannels are modulated by the suppressed signals, respectively, andfurther, are superposed on the main (L+R) channel signal so as to form acomposite signal. In receivers, therefore, after the left- andright-hand signals are reproduced from the main (L+R) channel signal andthe stereo subchannel (L-R) signal contained in the composite signal,and after the SAP signal is reproduced from the composite signal, it isnecessary to expand the (L-R) signal and the SAP signal which have beensuppressed for noise reduction.

In other words, the signal expansion processing is required for both the(L-R) signal and the SAP signal. On the other hand, the stereo sound andthe second audio program (SAP) will not simultaneously listened.Therefore, "triple audio signal" demodulating circuits have beenordinarily constructed such that either the first subchannel signal orthe second subchannel signal is selectively supplied to a DBX noisereduction decoder, and the signal restored to the original form by thedecoder is fed to a stereo multplexer or outputted as the SAP signal.

However, the selective application of the signal to the noise reductiondecoder has been determined by a manual switch. On the other hand, thetelevision broadcasting is not ceaselessly in the form of stereophonicor SAP broadcasting, and the times of the SAP broadcasting are less thanthe stereophonic broadcasting. The stereophonic broadcasting is so madethat it can be received as the monophonic broadcasting by the monophonicreceivers, and the stereophonic television receivers are constructedsuch that the TV viewer can without interruption listen even if thestereophonic broadcasting is changed to the monophonic one.

However, when only the stereo signal is transmitted without the SAPsignal, if the manual switch is operated to a SAP signal receivingposition, no sound is outputted from the television receivers. Further,when both the stereo signal and the SAP signals are transmitted and theSAP signal is selected by a TV viewer, if the SAP program is terminated,a sound will suddenly disappear from the TV receiver. In this case, hehas to operate the manual switch to a stereo broadcasting receivingposition. Thus, in the TV receiver equipped with the conventionalmultivoice signal demodulating circuit, no sound is outputted from thetermination of the SAP program to the operation of the manual switch.This would cause most TV viewers to be disoriented. In addition, theforced operation of the manual switch is troublesome to the TV viewers.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amultivoice signal switching circuit which can solve the above mentioneddrawbacks of the conventional multivoice signal demodulating circuits.

Another object of the present invention is to provide a multivoicesignal switching circuit which can be automatically changed to acondition of listening a main channel or first subchannel signal when asecond subchannel signal such as the SAP signal is not transmitted orhas been terminated.

Still another object of the present invention is to provide a multivoicesignal switching circuit for use in demodulators capable of reproducingnecessary signal from a multivoice signal containing a main channelsignal and first and second subchannel signals treated for noisereduction, which circuit is capable of selecting either of the first andsecond subchannel signals in accordance with a manual selection when apredetermined one of the two subchannel signals is contained in thereceived multivoice signal, and is adapted to automatically select thesubchannel signal other than the predetermined subchannel signal whenthe predetermined one is not contained in the received multivoicesignal.

A further object of the present invention is to provide a multivoicesignal demodulating circuit capable of reproducing necessary signal froma multivoice signal containing a main channel signal and first andsecond subchannel signals treated for noise reduction, in which circuiteither the first or second subchannel signal is selected and decoded inrespect of the noise reduction in accordance with a manual selectionwhen the predetermined one of the two subchannel signals is contained inthe received multivoice signal, and the subchannel signal other than thepredetermined subchannel signal is automatically selected and decoded inrespect of the noise reduction when the predetermined one is notcontained in the received multivoice signal.

The above and other objects of the present invention are accomplished bya multivoice signal switching circuit constructed in accordance with thepresent invention, which comprises means receiving a control signal forselectively outputting one of first and second channel signals extractedfrom a multivoice signal, means for sensing the second channel signal,and means connected to the sensing means and for forcedly putting theselective output means in a condition of outputting the first channelsignal irrespectively of the control signal, when the second channelsignal cannot be sensed.

Further, in accordance with the present invention there is provided acircuit for demodulating a multivoice signal containing a main channelsignal, a first subchannel signal, and a second subchannel signal, thelatter two signals being treated for noise reduction. The demodulatingcircuit comprises first circuit means for extracting a main channelsignal from the multivoice signal, second circuit means for extracting afirst subchannel signal from the multivoice signal, third circuit meansfor extracting a second subchannel signal from the multivoice signal,switching circuit means receiving the first and second subchannelsignals for selectively outputting one of the two subchannel signals,decoder means connected to an output of the switch circuit means torestore the original signal from the noise reduction treated signal, andoutput circuit means receiving the outputs of the first circuit meansand the decoder means for generating an audio signal. The switch circuitmeans has a manual switch for selecting either of the first and secondsubchannel signals, and the switch circuit means is adapted toinvalidate selection of the second subchannel signal by the manualswitch when the second subchannel signal cannot be sensed in themultivoice signal.

With the above circuit construction, when the second subchannel signalis not transmitted or has been terminated, the switch circuit meansoperates to invalidate the selection of the second subchannel signal bythe manual switch. Therefore, even if the manual switch is in or isbrought into a second subchannel selection position, the circuit isautomatically set so as to output the first subchannel signal.

The above and other objects, features and advantages of the presentinvention will be apparent from the following description of preferredembodiments of the invention with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a conventional multivoice signaldemodulating circuit;

FIG. 2 is a block diagram showing a multivoice signal demodulatingcircuit incorporating one embodiment of the switching circuit inaccordance with the present invention;

FIG. 3 is a circuit diagram showing the (L-R) signal demodulator and the(L-R)/SAP signal switching circuit shown in FIG. 2; and

FIG. 4 is a circuit diagram showing the auto-switching stereomultiplexer shown in FIG. 2 and a circuit for generating a controlsignal which is supplied to the switching circuit shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a conventional circuit fordemodulating the multivoice signal containing the main (L+R) channelsignal, the first subchannel (L-R) signal and the second subchannel(SAP) signal, the two subchannel signals being compressed in the DBXnoise reduction method. The shown demodulating circuit comprises amultivoice signal input terminal connected to a low pass filter 10 and abandpass filter 12. The low pass filter 10 has the cutoff frequency of50 KHz so as to extract from the input signal a stereo composite signalincluding a base band (L+R) signal and a first subcarrier carrying the(L-R) signal. On the other hand, the bandpass filter 12 has the centerfrequency of 5f_(H) (where f_(H) is the frequency of a horizontalsynchronizing signal), so that a second subcarrier carrying the SAPsignal is extracted from the input multivoice signal.

The low pass filter 10 has an output connected to another low passfilter 14, a f_(H) pilot detector 16 and a (L-R) signal demodulator 18.The low pass filter 14 has the cutoff frequency of 15 KHz so as tosupply only the (L+R) signal to a (L+R) input of a stereo multiplexer 20which functions as a stereo decoder.

The f_(H) detector 16 detects the pilot signal of frequency f_(H)(15.734 KHz) and outputs a stereo indication signal to a lamp driver 22so as to cause it to light an associated LED diode 24. At the same time,the pilot detector 16 generates pulses having the same pulse repetitionfrequency as the frequency 2f_(H) (31.468 KHz) of the first subcarrierand in synchronism with the pilot signal. The pulses are supplied to the(L-R) signal demodulator 18. To the contrary, when the pilot signalcannot be detected, no stereo indication signal is fed to the lampdriver 22, and so, the LED diode 24 is not energized. In addition, theabove mentioned pulses are not supplied to the (L-R) signal demodulator.

With reference to the pulses of 2f_(H), the (L-R) signal demodulator 18detects the stereo composite signal to reproduce the compressed (L-R)signal, which is in turn outputted through a 15 KHz low pass filter 26to one input terminal 28A of a stereo/SAP manual switch 28.Incidentally, the low pass filter 26 can be omitted.

On the other hand, the bandpass filter 12 has an output connected torespective inputs of a SAP signal demodulator 30 and a 5f_(H) carrierdetector 22. This 5f_(H) carrier detector 32 operates to detect thesecond subcarrier of the frequency 5f_(H) (78.67 KHz). When the secondsubcarrier is detected, the carrier detector 32 generates an SAPindication signal to a lamp driver 34, so that a LED diode 36 isenergized by the driver 34. Further, the carrier detector 32 produces acarrier signal of 5f_(H), i.e., 78.67 KHz to the SAP signal demodulator30. However, when the 5f_(H) subcarrier is not detected, it does notoutput the SAP indication signal, and so, the LED diode 36 is notlighted. In addition, no 5f_(H) carrier signal is supplied to the SAPsignal demodulator 30.

This SAP signal demodulator 30 detects the output signal of the 5f_(H)bandpass filter 12 by the carrier signal of 5f_(H) (78.67 KHz) so as toreproduce the compressed SAP signal, which is then fed through a 15 KHzlow pass filter 38 to the other input terminal 38B of the stereo/SAPmanual switch 28. The low pass filter 38 can be omitted, similarly tothe filter 26.

The stereo/SAP manual switch 28 has a common terminal connected to aninput of a DBX noise reduction decoder 40. Therefore, this decoder 40operates to expand the (L-R) signal or the SAP signal outputted from themanual switch 28, which signals are compressed in the DBX noisereduction method. The expanded (L-R) or SAP signal is connected to acommon terminal of another stereo/SAP manual switch 42, whose one fixedterminal 42A is connected to a (L-R) input of the stereo multiplexer 20.The other fixed terminal 42B of the switch 42 constitutes an SAP signaloutput terminal.

The two switches 28 and 42 are interlocked by a broken line in FIG. 1,so that the fixed terminal 28A is selected in the switch 28 when thefixed terminal 42A is selected in the switch 42.

In the above mentioned multivoice signal demodulator, when only thestereo signal is transmitted without the SAP signal, if the manualswitches 28 and 42 are operated to the SAP signal receiving positions28B and 42B, no sound is outputted from the television receiver.Further, when both the stereo signal and the SAP signal are transmittedand the SAP signal is selected by a TV viewer, if the SAP program hasbeen terminated, a sound will suddenly disappear from the TV receiver.In this case, he has to operate the manual switches 28 and 42 to astereo broadcasting receiving positions 28A and 42A.

Turning to FIG. 2, there is shown one embodiment of a multivoice signaldemodulating circuit incorporating an switching circuit in accordancewith the present invention. In FIG. 2, elements similar to those of thecircuit shown in FIG. 1 are given the same Reference Numerals, and theirexplanation will be omitted.

As will be apparent from the comparison between FIGS. 1 and 2, thecircuit shown in FIG. 2 comprises an analog switch circuit 50 in placeof the stereo/SAP manual switches 28 and 42, and the switch circuit 50is associated with a manual switch 52. In addition, an auto-switchingstereo multiplexer 54 is used instead of the stereo multiplexer 20.

The analog switch circuit 50 has two inputs connected to the outputs ofthe low pass filters 26 and 38, respectively, and is controlled toselect a manual switch operation or an auto-switching operation on thebasis of the reception indication signals from the f_(H) pilot detector16 and the 5f_(H) carrier detector 32.

Specifically, when the reception indication signal from the 5f_(H)carrier detector 32 indicates that the SAP signal is received, theanalog switch circuit 50 operates to couple the input of the DBX noisereduction decoder 40 to one of the low pass filters 26 and 38 which isdesignated by the manual switch 52. On the other hand, when the SAPsignal is not received, the analog switch circuit 50 operate to connectthe output of the filter 26 to the DBX noise reduction decoder 40,irrespectively of the condition of the manual switch 52. Namely, in thiscase, the selection by the manual switch is invalidated.

The auto-switching stereo multiplexer 54 is controlled by the analogswitch circuit 50 to perform the ordinary stereo demodulation when thestereo (L-R) signal is selected by the analog switch circuit 50, so thatthe L and R signals are respectively outputted from a pair of audiooutputs of the multiplexer 54. If the SAP signal is selected by theswitch circuit 50, the multiplexer 54 is controlled to output the SAPsignal from both the pair of audio outputs. Furthermore, the multiplexer54 is also controlled by the reception indication signal of the f_(H)pilot detector 18 which is supplied through the analog switch circuit50. Namely, when the output of the filter 26 is selected by the analogswitch circuit 50, if the television audio signal is a monophonicsignal, the multiplexer operates to output the (L+R) signal from thefilter 14 to both the pair of audio outputs.

Thus, the multivoice signal demodulating circuit as shown in FIG. 2operates as follow:

(a) When both the stereo signal and the SAP signal are transmitted,

a desired signal is selected from between the stereo and SAP signals bythe operation of the manual switch 52.

(b) When only the SAP signal is transmitted as the subchannel signal,i.e., when the monophonic signal and the SAP signal are transmitted,

a desired signal is selected from between the monophonic and SAP signalsby the operation of the manual switch 52.

(c) When only the stereo signal is transmitted and the SAP signal is nottransmitted,

the stereo signal is selected without regard to the condition of themanual switch 52. Namely, the (L-R) signal output of the filter 26 isinputted to the DBX noise reduction decoder 40 by the analog switchcircuit 50, and the (L+R) signal and the expanded (L-R) signal areapplied to the stereo multiplexer 54 so that it outputs the L and Rsignals from the pair of audio output terminals.

(d) When only the monophonic signal is transmitted and the SAP signal isnot transmitted,

the output of the filter 26 is connected to the DBX noise reductiondecoder 40 by the analog switch circuit 50, but since it does notinclude the (L-R) signal, only the (L+R) signal is applied to the stereomultiplexer 54. Therefore, the (L+R) signal, i.e., the monophonic signalis supplied from the pair of audio output terminals of the multiplexer.

Referring to FIG. 3, there is shown one example of the (L-R) signaldemodulator 18 and the analog switch circuit 50.

The (L-R) signal demodulator 18 includes a transistor Q₁ having a baseconnected to the output of the low pass filter 10, and anothertransistor Q₂ whose base is connected to a bias voltage V_(bias).Emitters of these transistors Q₁ and Q₂ are respectively connectedthrough resistors to a common constant current source. A collector ofthe transistor Q₁ is connected to common connected emitters oftransistors Q₃ and Q₄, while a collector of the transistor Q₂ isconnected to common connected emitters of transistors Q₅ and Q₆.

Of the transistors Q₃, Q₄, Q₅ and Q₆, the transistors Q₃ and Q₆ havecommon-connected bases, which receive non-inverted pulses of thefrequency 2f_(H) from the f_(H) pilot detector 16. The transistors Q₄and Q₅ also have common-connected bases which are supplied with invertedpulses of the frequency 2f_(H). Collectors of the transistors Q₃ and Q₄are common-connected to a DC source voltage V_(cc), and collectors ofthe transistors Q₄ and Q₆ are also common-connected to the DC sourcevoltage V_(cc) through a resistor and an active load including a pair oftransistors Q₇ and Q₈ connected in the form of a Darlington circuit.

As seen from the above, the (L-R) signal demodulator 18 is in thecircuit form of a double balanced differential amplifier.

The Darlington circuit consisting of the transistors Q₇ and Q₈ isconnected to another Darlington circuit composed of transistors Q₉ andQ₁₀, so that a current mirror circuit is formed.

On the other hand, the SAP signal demodulator 30 receiving the SAPmodulation signal from the 5f_(H) bandpass filter 12, is formed by forexample a frequency modulation detector, and is connected at its outputto a base of a transistor Q₁₁. This transistor Q₁₁ has an emittergrounded through a resistor and a collector connected to the DC sourcevoltage V_(cc) through a resistor and an active load formed by aDarlington circuit of transistors Q₁₂ and Q₁₃. This Darlington circuitis connected to another Darlington circuit having a pair of transistorsQ₁₄ and Q₁₅, so that a current mirror circuit is formed.

In the two current mirror circuits mentioned above, emitters of thetransistors Q₁₀ and Q₁₅ are common-connected to a ground resistor R, andalso to a bases of an output transistor Q₀. This output transistor has acollector connected through a resistor to the DC source voltage V_(cc),and an emitter of the transistor Q₀ is connected to a constant currentsource so as to form an emitter follower. Therefore, this emitter of thetransistor is connected to an output terminal H.

Further, as shown in FIG. 3, the collectors of the transistors Q₉, Q₁₀and Q₁₄, Q₁₅ of the two current mirror circuits are connected to abalanced differential circuit type of switch circuit 56 which comprisesfour transistors Q₁₆, Q₁₇, Q₁₈ and Q₁₉ connected as shown. Namely, acollector of the transistor Q₁₆ is connected to a point A, i.e., thecollectors of the transistors Q₉ and Q₁₀, and a collector of thetransistor Q₁₈ is connected to a point B, i.e., the collectors of thetransistors Q₁₄ and Q₁₅. Collectors of the remaining transistors Q₁₇ andQ₁₉ are common-connected to the DC source voltage V_(cc).

In addition, the transistors Q₁₆ and Q₁₉ are common-connected at theirbases, which are connected to a switching control input terminal D. Onthe other hand, the transistors Q₁₇ and Q₁₈ have their basescommon-connected to a biasing voltage V_(bias). Emitters of thetransistors Q₁₆ and Q₁₇ are connected to a constant current source,while emitters of the transistors Q₁₈ and Q₁₉ are connected to anotherconstant current source.

The circuit as shown in FIG. 3 operates as follows:

The (L-R) signal demodulator 18 in the form of the double balanceddifferential amplifier demodulates the composite signal from the lowpass filter 10 on the basis of the pulses of the frequency 2f_(H) fromthe f_(H) pilot detector 16, so as to reproduce the (L-R) signal carriedon the subcarrier of the frequency 2f_(H). This reproduced (L-R) signalflows through the active load formed by the Darlington circuit includingthe transistors Q₇ and Q₈. As the result of the current mirror effect, asimilar current flows through the Darlington circuit composed of thetransistors Q₉ and Q₁₀, so that the (L-R) signal voltage appear acrossthe grounding resistor R, and therefore, is applied to the base of theoutput transistor Q₀.

On the other hand, the SAP signal reproduced by the SAP signaldemodulator is applied to the base of the transistor Q₁₁ so as toproduce the corresponding collector current flowing through thetransistor Q₁₁. As a result, the SAP signal current flows through theactive load composed of the Darlington circuit including the transistorsQ₁₂ and Q₁₃, and due to the current mirror effect a similar currentflows through the Darlington circuit of the transistors Q₁₄ and Q₁₅.Therefore, the SAP signal voltage appears across the grounding resistorR and is applied to the base of the output transistor Q₀.

Thus, the (L-R) signal and the SAP signal are collided at a point C,i.e., at the base of the output transistor Q₀. But, this collision isprevented by the balanced differential circuit type switch circuit 56,and also, one of the two signals is selectively applied to thetransistor Q₀.

Namely, when the bases of the transistors Q₁₆ and Q₁₉, i.e., theswitching control input D is applied with a voltage higher than thebiasing voltage of the transistors Q₁₇ and Q₁₈ (SAP mode), thetransistors Q₁₆ and Q₁₉ are turned on, while the transistors Q₁₇ and Q₁₈are maintained in an OFF condition. Therefore, since the point A isgrounded through the transistor Q₁₆, the (L-R) signal does not flowthrough the transistor Q₁₀, and accordingly, does not appear on thepoint C. On the other hand, since the point B is not grounded, the SAPsignal flows through the transistor Q₁₅ and appears on the point C.Therefore, only the SAP signal is outputted through transistor Q₀ to theterminal D.

If the switching control input D is applied with a voltage lower thanthe biasing voltage of the transistor Q₁₇ and Q₁₈ (stereo mode), thetransistors Q₁₇ and Q₁₈ are turned on, and the transistors Q₁₆ and Q₁₉are brought into an OFF condition. Therefore, since the point B isgrounded through the transistor Q₁₈, the SAP signal is applied to thepoint C. Accordingly, only the SAP signal is applied to the point C andthen outputted through the transistor Q₀.

In the operation as mentioned above, either the (L-R) signal or the SAPsignal is selectively outputted from the terminal D.

Turning to FIG. 4, there is shown one example of the auto-switchingstereo multiplexer 54 shown in FIG. 2 and an associated circuit forgenerating a switching control signal which is supplied to the terminalD of the switch circuit 56 shown in FIG. 3 and to the auto-switchingstereo multiplexer.

The SAP indication signal output of the 5f_(H) carrier detector 32 isconnected to a base of a transistor Q₂₀. As seen from the drawing, thistransistor Q₂₀ is connected to another transistor Q₂₁ to form adifferential circuit. The base of the transistor Q₂₁ is connected to aconstant bias voltage V_(bias), and emitters of these transistors Q₂₀and Q₂₁ are common-connected and grounded through a constant currentsource. A collector of the transistor Q₂₀ is connected to a DC sourcevoltage V_(cc) through a transistor Q₂₂ which is connected to atransistor Q₂₃ to form a current mirror circuit. On the other hand, acollector of the transistor Q₂₁ is connected to the DC source voltageV_(cc) through a transistor Q₂₄ connected to a transistor Q₂₅ to formanother current mirror circuit.

The shown circuit includes a stereo/SAP switch terminal J connectedthrough the manual switch 52 to the DC source voltage V_(cc). Thisterminal J is also connected through series-connected resistors R₁, R₂and R₃ to a first biasing voltage such as 5.2 V. The connection betweenthe resistors R₁ and R₂ is connected to a base of a transistor Q₂₆,which is connected to a transistor Q₂₇ as shown in FIG. 4 to form adifferential circuit. Emitters of these transistors Q₂₆ and Q₂₇ arecommon-connected to the collector of the transistor Q₂₅. The collectorof the transistor Q₂₆ is connected through a resistor R₄ to a biasvoltage V_(bias), and also connected to an SAP selection input of astereo matrix circuit 58.

The collector of the transistor Q₂₇ is connected to the same biasvoltage V_(bias) through a transistor Q₂₉, which is connected to atransistor Q₃₀ to form a current mirror circuit. A collector of thetransistor Q₃₀ is connected to the collector of the transistor Q₂₀.Thus, the transistor Q₂₂ and hence the transistor Q₂₃ are brought intoON condition if either the transistor Q₂₀ or Q₃₀ is rendered conductive.

The base of the transistor Q₂₇ is connected through a resistor to asecond bias voltage such as 4.7 V, and also connected to an emitter of atransistor Q₂₈. This transistor 28 is connected at its collector to theDC source voltage V_(cc) and at its base to the connection between theresistors R₂ and R₃.

The collector of the transistor Q₂₃ is connected to a stereo/monophonicswitch circuit 60 which is adapted to receive the stereo indicationsignal output of the f_(H) pilot detector 16. This stereo/monophonicswitch circuit 60 has a stereo output connected through a resistor R₅ tothe bias voltage V_(bias) and also connected to a stereo selection inputof the stereo matrix circuit 58. The switch circuit 60 further has amonophonic output connected through a resistor R₆ to the bias voltageV_(bias) and also coupled to a monophonic selection input of the stereomatrix circuit 58.

The stereo matrix circuit 58 has a (L+R) input connected to the low passfilter 14 and a (L-R)/SAP input connected to the output of the DBX noisereduction decoder 40. Further, the matrix circuit 58 has a pair of audiooutputs L and R. When the SAP selection input is put at a high voltagelevel, the matrix circuit 58 operates to output from the pair of audiooutputs L and R the SAP audio signal which is supplied to the (L-R)/SAPinput. When the SAP selection input is at a low voltage level and thestereo selection input is brought to a high voltage level, it operatesto reproduce left- and right-hand audio signals from the (L+R) signaland (L-R) signal and then to output these left- and right-hand audiosignals to the pair of audio outputs L and R. In addition, when themonophonic selection input is applied with a high voltage level, thematrix circuit 58 operates to output from the pair of audio outputs Land R the same (L+R) signal supplied to the (L+R) input.

In the above mentioned circuit, when the manual switch 52 is closed, thestereo/SAP switch terminal J is rendered to the DC source voltage V_(cc)(SAP mode). At this time, the bases of the transistors Q₂₆ and Q₂₇ arerespectively applied with bias voltages expressed as follows: ##EQU1##where V_(BQ26) is the base voltage of the transistor Q₂₆ ;

V_(BQ27) is the base voltage of the transistor Q₂₉ ; and

V_(BEQ28) is the base-emitter voltage of the transistor Q₂₈.

At this time, the resistances of the resistors R₁, R₂ and R₃ aredetermined to establish the relation V_(BQ26) <<V_(BQ27). For example,assuming that R₁ =R₂ =5KΩ and R₃ =10KΩ, the relation of V_(BQ26) =6.9 Vand V_(BQ26) =7.9 V can be obtained under the condition of V_(cc) =12 V.

Under this condition, the transistor Q₂₇ is off and the transistor Q₂₆is ON. Namely, most of the current flowing through the transistor Q₂₅will flow through the transistor Q₂₆, so that the potential at the pointE become high. As a result, the high voltage level is applied to the SAPselection input of the matrix circuit 58, so as to instruct theselection of the SAP mode.

The current flowing through the transistor Q₂₅ is controlled through thetransistor Q₂₄ by the differential circuit constituted of thetransistors Q₂₀ and Q₂₁. Now, assuming that the SAP indication signalfrom the 5f_(H) carrier detector 32 is at a low level (SAP signalreception), the transistor Q₂₀ is OFF and the transistor Q₂₁ is ON. Atthis time, therefore, the transistor Q₂₅ is put in a condition ofsupplying the sufficient current to the transistors Q₂₆ and Q₂₇. But,when the SAP signal is not received and then the 5f_(H) carrier detectoroutputs a signal having a voltage sufficiently higher than the base biasvoltage of the transistor Q₂₁, the transistor Q₂₀ is turned on and thetransistor Q₂₁ becomes off. Accordingly, a current does not flow throughthe transistor Q₂₆ irrespectively of the condition of the stereo/SAPmanual switch 52. On the other hand, since the transistor Q₂₀ is in ONcondition, the transistor Q₂₂ is put in ON condition, and therefore, thetransistor Q₂₃ is brought into a condition capable of supplying acurrent to the stereo/monophonic switch circuit 60. Accordingly, even ifthe manual switch 52 is operated to select the SAP mode, the stereo modeor the monophonic mode is selected by the switch circuit 60.

When the stereo mode is selected, the point F takes a high potential. Onthe other hand, when the monophonic mode is selected, the point Gbecomes a high potential. As mentioned above, the matrix circuit 58determines the operation mode on the bases of the potentials at thepoints E, F, and G.

When the manual switch 52 is opened to put the stereo/SAP switchterminal J in a floating condition, the base potential of the transistorQ₂₆ is brought to about 5.2 V, and the transistor Q₂₇ takes the basebias of about 4.7 V. In this condition, the transistor Q₂₇ is turned onand the transistor Q₂₆ is turned off. At this time, if the transistorQ₂₅ is in ON condition (the SAP signal reception condition), the currentflows through the transistor Q₂₉. As a result, the current is suppliedthrough the transistor Q₂₃ to the switch circuit 60, so that it takeseither the stereo mode or the monophonic mode in accordance with theoutput of the detector 16. In addition, when the transistor Q₂₅ is putin OFF condition (i.e., when the SAP signal is not received), thetransistor Q₂₉ is not supplied with the current from the transistor Q₂₇.However, since the transistor Q₂₂ is rendered conductive by thetransistor Q₂₀, so that the current flows the transistor Q₂₃ so as toput the switch circuit 60 in an operable condition.

As will be apparent from the above explanation, a stereo/SAP switchingoperation can be achieved by connecting the point E to the switchcontrol terminal D of the switch circuit 56 shown in FIG. 3 andconnecting the DBX noise reduction decoder 40 between the output H ofthe FIG. 3 circuit and the (L-R)/SAP input of the matrix circuit 58.

As seen from the above, the transistors Q₂₀, Q₂₁, Q₂₄, Q₂₅, Q₂₆, Q₂₇ andQ₂₈ and the associated passive elements and voltage sources, surroundedby a dotted line in FIG. 4, constitutes a circuit for generating aswitch control signal to be supplied to the switch control input D ofthe switch circuit 56 shown in FIG. 3.

The invention has thus been shown and described with reference tospecific embodiments. However, it should be noted that the invention isin no way limited to the details of the illustrated structures butchanges and modifications may be made withi the scope of the appendedclaims.

I claim:
 1. A multivoice signal switching circuit for treating amultivoice signal including a main channel and first and secondsubchannels, the first subchannel being assigned to stereophonicinformation and the second subchannel being assigned to second languageinformation, comprising means receiving a control signal for selectivelyoutputting one of the first and second subchannel signals extracted fromsaid multivoice signal, means for sensing the second subchannel signal,and means connected to the sensing means and for controlling theselectively outputting means so as to force said selectively outputtingmeans to a condition of outputting the first subchannel signalirrespective of the control signal, when the second subchannel signalcannot be sensed.
 2. A circuit claimed in claim 1 wherein theselectively outputting means includes an analog switch circuit having afirst current mirror circuit connected to receive at its input currentpath the first subchannel signal, and a second current mirror circuitconnected to receive at its input current path the second subchannelsignal, an output transistor having an input connected commonly to oneend of each of respective output paths of the first and second currentmirror circuits, and means connected to another end of each of theoutput paths of the first and second current mirror circuits forselectively shunting either the first or second current mirror circuitin accordance with selection of the two subchannel signals.
 3. A circuitclaimed in claim 2 wherein the shunting means includes a balanceddifferential circuit having a control input and two outputs connected tothe other ends of the output paths of the first and second currentmirror circuits, respectively, the two outputs of the differentialcircuit being selectively grounded in accordance with the condition ofthe control input.
 4. A circuit claimed in claim 3 wherein the means forcontrolling the selectively outputting means includes a firstdifferential circuit having an input connected to the sensing means andtwo output paths which are alternatively rendered conductive inaccordance with the condition of the input, and a second differentialcircuit connected to one of the two output paths of the firstdifferential circuit so that it is put in an operable condition when theone output path of the first differential circuit is renderedconductive, the second differential circuit having an input connected toa manual switch for selection of the first and second subchannelsignals, the second differential circuit also having two output paths,one of which is rendered conductive when the manual switch is in a firstcondition, and the other of which is rendered conductive when the manualswitch is in a second condition, the one output path of the seconddifferential circuit being adapted to be connected to the control inputof the shunting means.
 5. A circuit claimed in claim 4 wherein thesensing means is a detector for detecting a subcarrier for the secondsubchannel signal.
 6. A circuit for demodulating a multivoice signalcontaining a main channel signal, a first subchannel signal, and asecond subchannel signal, the latter two signals being treated for noisereduction, comprising first circuit means for extracting a main channelsignal from the multivoice signal, second circuit means for extracting afirst subchannel signal from the multivoice signal, third circuit meansfor extracting a second subchannel signal from the multivoice signal,switch circuit means receiving the first and second subchannel signalsfor selectively outputting one of the two subchannel signals, decodermeans connected to an output of the switch circuit means to restore theoriginal signal from the noise reduction treated signal, and outputcircuit means receiving the outputs of the first circuit means and thedecoder means for generating an audio signal, the switch circuit meanshaving a manual switch for selecting one of either the first and secondsubchannel signals, the switch circuit means further including means toinvalidate selection of the second subchannel signal by the manualswitch when the second subchannel signal cannot be sensed in themultivoice signal.
 7. A circuit claimed in claim 6 wherein the switchingmeans includes an analog switch circuit having a first current mirrorcircuit receiving at its input current path the first subchannel signal,and a second current mirror circuit receiving at its input current paththe second subchannel signal, an output transistor having an inputconnected commonly to one end of each of respective output paths of thefirst and second current mirror circuits and an output connected to thedecoder means, and means connected to the other end of each of theoutput paths of the first and second current mirror circuits forselectively shunting either the first or second current mirror circuitin accordance with selection of the two subchannel signals.
 8. A circuitclaimed in claim 7 wherein the shunting means includes a balanceddifferential circuit having a control input and two outputs connected tothe other ends of the output paths of the first and second currentmirror circuits, respectively, the two outputs of the differentialcircuit being selectively grounded in accordance with selection of thetwo subchannel signals.
 9. A circuit claimed in claim 8 wherein theswitch circuit also includes a first differential circuit having aninput connected to the third circuit means and two output paths whichare alternatively rendered conductive in accordance with the conditionof the input, and a second differential circuit connected to one of thetwo output paths of the first differential circuit so that it is put inan operable condition when the one output path of the first differentialcircuit is rendered conductive, the second differential circuit havingan input connected to a manual switch for selection of the first andsecond subchannel signals and two output paths, one of which is renderedconductive when the manual switch is in a first condition, and the otherof which is rendered conductive when the manual switch is in a secondcondition, the one output path of the second differential circuit beingadapted to be connected to the control input of the shunting means. 10.A circuit claimed in claim 6 wherein the main channel signal, the firstsubchannel signal and the second subchannel signal are a (L+R) signal, a(L-R) signal and an SAP signal, respectively, and wherein the firstcircuit means includes a low pass filter extracting the (L+R) signalfrom the multivoice signal, the second circuit means includes a pilotdetector for detecting a pilot signal for the (L-R) signal from themultivoice signal and generating a (L-R) signal reception signal, and a(L-R) signal demodulator controlled by the pilot detector to reproducethe (L-R) signal from the multivoice signal; and the third circuit meansincludes a SAP subcarrier detector for detecting a subcarrier for theSAP signal from the multivoice signal and generating a SAP signalreception signal, and a SAP signal demodulator controlled by thesubcarrier detector to reproduce the SAP signal from the multivoicesignal.
 11. A circuit claimed in claim 10 wherein the switch circuitincludes an analog switch circuit having a first current mirror circuitconnected to receive at its input current path the (L-R) signal, and asecond current mirror circuit connected to receive at its input currentpath the SAP signal, an output transistor having an input connectedcommonly to one end of each of respective output paths of the first andsecond current mirror circuits, and means connected to the other end ofeach of the output paths of the first and second current mirror circuitsfor selectively shunting either the first or second current mirrorcircuit in accordance with selection of the two subchannel signals. 12.A circuit claimed in claim 11 wherein the shunting means includes abalanced differential circuit having a control input and two outputsconnected to the other end of each of the output paths of the first andsecond current mirror circuits, respectively, the two outputs of thebalanced differential circuit being selectively grounded according tothe condition of a control input.
 13. A circuit claimed in claim 12wherein the balanced differential circuit includes a first transistorhaving a collector connected to the other end of the output path of thefirst current mirror circuit, a second transistor connected to the firsttransistor to form a first differential circuit, a third transistorhaving a collector connected to the other end of the output path of thesecond current mirror circuit, and a fourth transistor connected to thethird transistor to form a second differential circuit, the first andfourth transistors being common-connected at their bases to the controlinput, and the second and third transistors being common-connected to apredetermined bias voltage, so that the first and third transistors arealternatively turned on in accordance with the voltage applied to thecontrol input.
 14. A circuit claimed in claim 12 wherein the switchcircuit includes a first differential circuit having an input connectedto the subcarrier detector and two output paths which are alternativelyrendered conductive in accordance with the condition of the input, and asecond differential circuit connected to one of the two output paths ofthe first differential circuit so that it is put in an operablecondition when the one output path of the first differential circuit isrendered conductive, the second differential circuit having an inputconnected to a manual switch for selection of the first and secondsubchannel signals, the second differential circuit also having twooutput paths, one of which is rendered conductive when the manual switchis in a first condition, and the other of which is rendered conductivewhen the manual switch is in a second condition, the one output path ofthe second differential circuit being adapted to be connected to thecontrol input of the shunting means.
 15. A circuit claimed in claim 14wherein the first differential circuit includes a first transistorhaving a base connected to the subcarrier detector, and a secondtransistor connected to the first transistor to form a differentialcircuit, the second transistor having a base connected to apredetermined bias voltage and a collector connected to a DC sourcevoltage through one of two current paths of a third current mirrorcircuit, so that the first and second transistors are alternatively puton in accordance with the base potential of the first transistor, andwherein the second differential circuit includes third and fourthtransistors connected to form another differential circuit, the emittersof the third and fourth transistors being common-connected to the othercurrent path of the third current mirror circuit, the base of the thirdtransistor being connected through a first resistor to a first biasvoltage and also through second and third resistors and the manualswitch to the DC source voltage, the collector of the third transistorbeing connected to a control input of the balanced differential switchcircuit and also connected through a fourth resistor to a bias voltage,and the base of the fourth transistor being connected through a resistorto a second bias voltage.
 16. A circuit claimed in claim 15 wherein thecollector of the fourth transistor is connected through one of twocurrent paths of a fourth current mirror circuit to said first biasvoltage, the other current path of the fourth current mirror circuit andthe first transistor being connected to one of two current paths of afifth current mirror circuit, the other current path of the fifthcurrent mirror circuit being connected to a stereo/mono switch circuitwhose output is connected to a stereo matrix circuit.
 17. A switchingcircuit for treating a multivoice signal including a main channel andfirst and second subchannels for enabling both stereophonic informationtransmission and bilingual information transmission, the main channeland the first subchannel being used for stereophonic information, andthe main channel and the second subchannel being respectively assignedto a first language and a second language for the bilingual information,the switching circuit comprising a first means responsive to a controlsignal for enabling one selected from the group consisting of thestereophonic reproduction, the first language reproduction and thesecond language reproduction, a second means sensing a second subchannelsignal extracted from the multivoice signal, and a third means connectedto the second means for controlling the first means so as to force saidfirst means into a condition for enabling only either the stereophonicreproduction or the first language reproduction irrespective of thecontrol signal when the second channel signal is not sensed.
 18. Acircuit claimed in claim 17 wherein the first means includes an analogswitch circuit having a first current mirror circuit connected toreceive at its input current path a first subchannel signal extractedfrom the multivoice signal, and a second current mirror circuitconnected to receive at its input current path a second subchannelsignal, an output transistor having an input connected commonly to oneend of each of respective output paths of the first and second currentmirror circuits, and means connected to the other end of each of theoutput paths of the first and second current mirror circuits forselectively shunting either the first or second current mirror circuitin accordance with selection of the two subchannel signals.
 19. Acircuit claimed in claim 18 wherein the shunting means includes abalanced differential circuit having a control input and two outputsconnected to the other end of each of the output paths of the first andsecond current mirror circuits, respectively, the two outputs of thedifferential circuit being selectively grounded in accordance with thecondition of the control input.
 20. A circuit claimed in claim 19wherein the third means includes a first differential circuit having aninput connected to the second means and two output paths which arealternatively rendered conductive in accordance with the condition ofthe input, and a second differential circuit connected to one of the twooutput paths of the first differential circuit so that it is put in anoperable condition when the one output path of the first differentialcircuit is rendered conductive, the second differential circuit havingan input connected to a manual switch for selection of the first andsecond subchannel signals, the second differential circuit also havingtwo output paths, one of which is rendered conductive when the manualswitch is in a first condition, and the other of which is renderedconductive when the manual switch is in a second condition, the oneoutput path of the second differential circuit being adapted to beconnected to the control input of the shunting means.
 21. A circuit fordemodulating a multivoice signal containing a main channel signal, afirst subchannel signal, and a second subchannel signal, the latter twosignals being compressed in a DBX noise reduction system, the firstsubchannel being assigned to stereophonic information and the secondsubchannel being assigned to second language information, comprisingfirst circuit means for extracting a main channel signal from themultivoice signal, second circuit means for extracting a firstsubchannel signal from the multivoice signal, third circuit means forextracting a second subchannel signal from the multivoice signal,switching circuit means receiving the first and second subchannelsignals for selectively outputting one of the two subchannel signals,decoder means connected to an output of the switch circuit means torestore the original signal from the DBX noise reduction treated signal,and output circuit means receiving the outputs of the first circuitmeans and the decoder means for generating an audio signal, the switchcircuit means having a manual switch for selecting either of the firstand second subchannel signals, and the switch circuit means includingmeans for invalidating selection of the second subchannel signal by themanual switch when the second subchannel signal cannot be sensed in themultivoice signal.
 22. A circuit for demodulating a multivoice signalcontaining a main channel for a (L+R) signal, a first subchannel for a(L-R) signal, and a second subchannel for an SAP signal comprising:a lowpass filter extracting the (L+R) signal from the multivoice signal; apilot detector for detecting a pilot signal for the (L-R) signal fromthe multivoice signal and generating a (L-R) signal reception signal; a(L-R) signal demodulator controlled by the pilot detector to reproducethe (L-R) signal from the multivoice signal; a SAP subcarrier detectorfor detecting a subcarrier for the SAP signal from the multivoice signaland generating a SAP signal reception signal; a SAP signal demodulatorcontrolled by the subcarrier detector to reproduce the SAP signal fromthe multivoice signal; a switching circuit means receiving the (L-R) andSAP signals for selectively outputting one of the subchannel signals; adecoder means connected to an output of the switch circuit means torestore the original signal from the noise reduction treated signal; andoutput circuit means receiving the outputs of the low pass filter andthe decoder means for generating an audio signal, the switch circuitmeans having a manual switch for selecting either of the (L-R) and SAPsignals, and further includes means for invalidating selection of theSAP signal by the manual switch when the SAP signal cannot be sensed inthe multivoice signal.
 23. A circuit for demodulating a multivoicesignal containing a main channel for a (L+R) signal, a first subchannelfor a (L-R) signal, and a second subchannel for an SAP signal, the (L-R)signal and the SAP signal being treated to suppress, comprising:a lowpass filter extracting the (L+R) signal from the multivoice signal; apilot detector for detecting a pilot signal for the (L-R) signal fromthe multivoice signal and generating a (L-R) signal reception signal; a(L-R) signal demodulator controlled by the pilot detector to reproducethe (L-R) signal from the multivoice signal; a SAP subcarrier detectorfor detecting a subcarrier for the SAP signal from the multivoice signaland generating a SAP signal reception signal; a SAP signal demodulatorcontrolled by the subcarrier detector to reproduce the SAP signal fromthe multivoice signal; an analog switch circuit having a first currentmirror circuit connected to receive at its input current path the (L-R)signal, and a second current mirror circuit connected to receive at itsinput current path the SAP signal, an output transistor having an inputconnected commonly to one end of each of respective output paths of thefirst and second mirror circuits; a balanced differential circuitincluding a first transistor having a collector connected to the otherend of the output paths of the first current mirror circuit, a secondtransistor connected to the first transistor to form a firstdifferential circuit, a third transistor having a collector connected tothe other end of the output paths of the second current mirror circuit,and a fourth transistor connected to the third transistor to form asecond differential circuit, the first and fourth transistors beingcommon-connected at their bases to a control input for selection of the(L-R) signal and the SAP signal, and the second and third transistorsbeing common-connected to a predetermined bias voltage, so that thefirst and third transistors are alternatively turned on in accordancewith the voltage applied to the control input; a decoder connected to anoutput of the output transistor of the analog switch circuit to restorethe original signal from the noise reduction treated signal; and anoutput circuit receiving the outputs of the low pass filter and thedecoder for generating an audio signal.
 24. A circuit for demodulating amultivoice signal containing a main channel for a (L+R) signal, a firstsubchannel for a (L-R) signal, and a second subchannel for an SAPsignal, the (L-R) signal and the SAP signal being suppressed for noisereduction, comprising:a low pass filter extracting the (L+R) signal fromthe multivoice signal; a pilot detector for detecting a pilot signal forthe (L-R) signal from the multivoice signal and generating a (L-R)signal reception signal; a (L-R) signal demodulator controlled by thepilot detector to reproduce the (L-R) signal from the multivoice signal;a SAP subcarrier detector for detecting a subcarrier for the SAP signalfrom the multivoice signal and generating a SAP signal reception signal;a SAP signal demodulator controlled by the subcarrier detector toreproduce the SAP signal from the multivoice signal; an analog switchcircuit having a first current mirror circuit connected to receive atits input current path the (L-R) signal, and a second current mirrorcircuit connected to receive at its input current path the SAP signal,an output transistor having an input connected commonly to one end ofeach of respective output paths of the first and second current mirrorcircuits; a balanced differential switch circuit including a firsttransistor having a collector connected to the other end of the outputpaths of the first current mirror circuit, a second transistor connectedto the first transistor to form a first differential circuit, a thirdtransistor having a collector connected to the other end of the outputpath of the second current mirror circuit, and a fourth transistorconnected to the third transistor to form a second differential circuit,the first and fourth transistors being common-connected at their basesto a control input for selection of the (L-R) signal and the SAP signal,and the second and third transistors being common-connected to apredetermined bias voltage, so that the first and third transistors arealternatively turned on in accordance with the voltage applied to thecontrol input; a decoder connected to an output of the output transistorof the analog switch circuit to restore the original signal from thenoise reduction treated signal; third differential circuit including afifth transistor having a base connected to the subcarrier detector, anda sixth transistor connected to the fifth transistor to form adifferential circuit, the sixth transistor having a base connected to apredetermined bias voltage and a collector connected to a DC sourcevoltage through one of two current paths of a third current mirrorcircuit, so that the fifth and sixth transistors are alternatively puton in accordance with the base potential of the fifth transistor; afourth differential circuit including seventh and eighth transistorsconnected to form another differential circuit, the emitters of theseventh and eighth transistors being common-connected to the othercurrent path of the third current mirror circuit, the base of theseventh transistor being connected through a first resistor to a firstbias voltage and also through second and third resistors and a manualswitch to the DC source voltage, the collector of the seventh transistorbeing connected to the control input of the balanced differential switchcircuit and also connected through a fourth resistor to a second biasvoltage, and the base of the eighth transistor being connected through aresistor to a third bias voltage, the collector of the eighth transistoris connected through one current path of a fourth current mirror circuitto said predetermined bias voltage, another current path of the fourthcurrent mirror circuit and the fifth transistor being connected to onecurrent path of a fifth current mirror circuit, another current path ofthe fifth current mirror circuit being connected to a stereo/mono switchcircuit; and a stereo matrix circuit receiving the (L+R) signal and theoutput of the decoder and controlled by an output of the stereo-monoswitch circuit and the potential of the collector of the seventhtransistor so as to form a pair of audio outputs in one of a monoralmode, a stereophonic mode and a SAP mode.